The rotational excitation of methanol by molecular hydrogen

Abstract

We have computed cross-sections and rate coefficients for the rotational excitation of A- and E-type methanol by molecular hydrogen. Calculations were performed for rotational transitions within the torsional ground state, ν= 0, and within the first and second excited torsional states, ν= 1 and ν= 2. For collisions of methanol with para-H2 in its rotational ground state, j2= 0, the methanol basis included rotational states j1≤ 15, thereby extending previous calculations, which included states j1≤ 9 only. For the first time, calculations have also been performed for ortho-H2 in its rotational ground state, j2= 1, although it was necessary to revert to the smaller methanol basis, j1≤ 9, owing to the coupling to states of molecular hydrogen with j2 > 0. The coupled states approximation was used in the production calculations, to generate the thermal rate coefficients at temperatures 10 ≤T≤ 200 K, but some limited comparisons, at a few collision energies, of cross-sections obtained using the full coupled channels (CC) method have been made. The propensities of the collisions, with respect to changes in the rotational quantum number, j1, and its projection, K, on the symmetry axis of the methanol molecule, were investigated. There are qualitative differences between the K propensities for collisions with ortho- and para-H2, which relate to the fact that the inelastic cross-sections tend to be significantly larger when ortho-H2 is the perturber.